'Damning evidence' ... An undated family photograph showing Baha Mousa - beaten to death while in British custody in Iraq - and his young son. Photograph: Reuters

Law professor AT Williams has won the Orwell book prize for political writing for his investigation into the killing of hotel receptionist Baha Mousa by British Army soldiers in Iraq, A Very British Killing, a book which judges said was "written in the spirit" of Orwell's journalism.

Wednesday evening's award ceremony also saw the late journalist Marie Colvin's collected journalism, On the Front Line, win the Orwell Special Prize. Organisers said that the work of Colvin – who died last year in a rocket attack in Syria – "reflects on the state of humanity and the endeavour of a profession conducted passionately right up until she was killed in February 2012".

Director of the Orwell prize Jean Seaton added: "Marie Colvin's life – like many journalists' – was abruptly and terribly closed as she was doing her job. The threats journalists face all over the world have gone up – yet we need their intelligence more than ever. But her work has been beautifully shaped in this book. A life given to holding the powerful to account."

Williams's A Very British Killing was named winner of the £3,000 book award ahead of Colvin, the former Bishop of Edinburgh Richard Holloway, Pankaj Mishra, Raja Shehadeh, Carmen Bugan and Clive Stafford Smith. Organisers called it a "chilling, gripping book" which "unearths damning evidence of what happened" to Mousa, the receptionist who on 15 September 2003 was arrested in Basra and taken to a military base, where guards and army visitors kicked, punched and humiliated him before he was beaten to death.

Judges Nikita Lalwani, Arifa Akbar and Bakewell said that Williams, a law professor at the University of Warwick and director of the Centre for Human Rights in Practice, "had the courage to take on a case that has already received so much press coverage and to turn it into something far bigger and more shocking than we understood it to be".

"He dissects and analyses with a clear-eyed determination to unpick the lies from the truths of this case, yet, for all its forensic detail, the book grips us emotionally, and has as keen a sense of storytelling as a horror story or courtroom drama. Ultimately, the greatest achievement of this incendiary, eloquent and angry book is that it humanises Mousa beyond the iconic and infamous figure he has become in his death. It was written in the spirit of Orwell's journalism," they said.

The Orwell book prize is intended to discover the work which comes closest to George Orwell's ambition "to make political writing into an art". Writing in the Guardian earlier this month, Williams wondered what Orwell himself "would have thought" about the Mousa case. "He wrote once that: 'It seems to me nonsense, in a period like our own, to think that one can avoid writing of such subjects.' His main target then was the evil of totalitarianism," wrote Williams. "But I would like to think his underlying aim was to challenge indifference to the suffering of others. That for me was the real devil which emerged amid the detail of my book."

Williams joins former winners of the prize including Francis Wheen, Fergal Keane and Tom Bingham.